Debug Info: define a DIRef template.

Specialize the constructors for DIRef<DIScope> and DIRef<DIType> to make sure
the Value is indeed a scope ref and a type ref.

Use DIScopeRef for DIScope::getContext and DIType::getContext and use DITypeRef
for getContainingType and getClassType.

DIScope::generateRef now returns a DIScopeRef instead of a "Value *" for
readability and type safety.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190418 91177308-0d34-0410-b5e6-96231b3b80d8